The most high-profile defector from North Korea has said he was told in 1996 that the secretive state had already developed nuclear weapons.

Hwang Yang-jop, a former tutor to North Korean leader Kim Jong-il, also said that Pyongyang signed a contract with Pakistan the same year to receive help in enriching uranium for its nuclear arsenal.

Mr Hwang's comments will add credence to United States intelligence reports that the North had developed a small number of nuclear weapons before it agreed to mothball its programme in 1994.

The collapse of that agreement last year triggered mounting concern about the North's nuclear ambitions, and speculation as to whether it would be able to increase its arsenal.

Mr Hwang was making his first public comments since he defected to South Korea in 1997.

"I heard from Kim Jong-il and others including Jun Byong-ho that nuclear weapons had been produced," Mr Hwang told a parliamentary forum in Seoul.

Mr Hwang said Jun Byong-ho was a secretary of the central committee of the ruling Korean Workers Party in charge of Pyongyang's nuclear weapons development.

Mr Hwang said that that the North Korean regime - which he described as "a ring of crime opposing democracy and infringing on human rights" - must be disarmed.

However, Mr Hwang noted that a conventional military assault on North Korea would be "almost impossible".

"The North has piled up on weapons of mass destruction and turned the whole country into a fortress," he said.

Mr Hwang's naming of Pakistan is likely to be embarrassing to that country's government.

However, the US indicated earlier this year that it was no longer worried by reports that North Korea and Pakistan had collaborated on nuclear weapons technology.

US Secretary of State Colin Powell said in April that Pakistani President Pervez Musharraf had assured him there were no further contacts of the kind referred to in a newspaper report which said Islamabad provided Pyongyang with gas centrifuges and equipment to make highly-enriched uranium.